High-permittivity ceramics make it possible to noticeably miniaturize passive microwave devices. These ceramics must fulfil the requirements of high permittivity, very low dielectric losses and an extremely low temperature dependence of permittivity to yield temperature-stable resonators. Recent progress has been made in understanding the physics of microwave ceramics, particularly the property-structure relationship, in developing new materials with higher permittivity, better temperature stability and considerably lower sintering temperature, and in making co-fired multilayer devices possible, as well as in developing new microwave devices such as oscillators, antennas and band-pass filters.

Microwave ceramics for resonators and filters

The crystallography of tungstenbronze-type like Ba6−3xR8+2xTi18O54 solid solutions for microwave dielectric materials applied for mobile phone and base station were studied. The crystal data and crystal structure of the fundamental lattice and supperlattice, and derivation of chemical formula and structural formula are summarized. Fourier map of the fundamental structure showed that oxygen ion positions of TiO6 octahedra were splitted. This fact proved the tilting of octahedra which is cause of superlattice. The mechanisms of microwave dielectric properties are summarized. Finally, guidelines for the design of the microwave dielectric materials are presented.

Science of tungstenbronze-type like Ba6–3xR8+2xTi18O54 (R=rare earth) microwave dielectric solid solutions

Synthesis, structural, magnetic and transport properties of layered perovskite-related titanates, niobates and tantalates of the type AnBnO3n+2, A′Ak−1BkO3k+1 and AmBm−1O3m

Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24

Ba 2− x Sm 4+2 x /3 Ti 9 O 26 ( x =0–0.3) was selected as the formula for BaO–Sm 2 O 3 –4.5TiO 2 ceramics based on the structure formula of the tungsten–bronze-type structure. The microstructure and microwave dielectric properties of Ba 2− x Sm 4+2 x/ 3 Ti 9 O 26 and the relationships between them were investigated in this study. The microstructure of Ba 2− x Sm 4+2 x /3 Ti 9 O 26 ceramics consists of a uniform matrix of primary BaO–Sm 2 O 3 –4TiO 2 and BaO–Sm 2 O 3 –5TiO 2 with small amounts of dispersed second phase, TiO 2 and Ba 2 Ti 9 O 20 . Excellent dielectric properties of e r =68–79, Q × f =11 000–12 500 GHz and good temperature stability at the resonant frequency ( τ f =0 ppm per °C) were obtained. The relationships between the microstructure, second phases and microwave dielectric properties were concerned. XRD patterns, energy dispersive X-ray spectrometer (EDS) analysis and scanning electron microscopy are also presented.

B. Volavsek

Papers

Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24

The Nd2O3·4TiO2 phase in the system Nd2O3-TiO2